Process heater and finned tube therefor



1966 s. A. GUERRIERI 3,269,363

PROCESS HEATER AND FINNED TUBE THEREFOR Filed Sept. 2, 1964 5 Sheets-Sheet 1 INVENTOR. SALVATORE A. GUER/P/ER/ A 7'TORNEV6 0, 1966 s. A. GUERRIERI 3,269,363

PROCESS HEATER AND FINNED TUBE THEREFOR Filed Sept. 2, 1964 5 Sheets-Sheet 2 FIG. 2

LO-I

092 FIG. 3

INVENTOR.

SALl/ATORE A. GUERR/ER/ Y //////72 BY ATTORNEVJ' Aug. 30, 1966 s. A. GUERRIERI PROCESS HEATER AND FINNED TUBE THEREFOR 5 Sheets$heet 5 Filed Sept. 2, 1964 FIG. 4

R O T N E W m ATTORNEYS United States Patent 3,269,363 PROCESS HEATE AND FINNED TUBE THEREFQR Salvatore A. Guerrieri, Rowayton, Conn, assignor to The Lummus Company, New York, N.Y., a corporation of Delaware Filed Sept. 2, 196a, Scr. No. 393,990 3 Claims. or. 122-356) This invention relates, in general, to a new and improved process heater and finned radiant tubes therefor and, more particularly, to a process heater which employs improved tubes through which process fluid passes while being heated within a chamber, which process heater is capable of achieving more uniform heating of the process fluid and is designed for closer spacing of the tubes within the chamber so as to cut the overall cost of the process heater.

In high-performance pyrolysis heaters and other fired tubular reactors, it is the aim of designers of such equipment to obtain, as nearly as possible, uniform heat flux around the tube circumference. At present, this ideal is approached more or less by arranging the tubes in a single row along the center line of a furnace and introducing heat through the two walls of the furnace parallel to the plane of the tubes so that the tubes are heated from two sides. Despite this arrangement, the heat flux around the tubes varies from a maximum along the lines directly opposite the firing walls to a minimum along the lines ninety degrees therefrom.

This heat flux variation is further increased by the close spacing of the tubes. If the tubes are very close to one another, the points ninety degrees from the points directly opposite the firing walls are further prevented from receiving reradiated and directly radiated heat from the firing walls by the adjacent tubes.

Therefore, it is the general object of this invention to provide a new and improved process heater and radiant tubes therefor which is less expensive to manufacture and more efficient in operation.

Another object of this invention is the provision of a new and improved process heater in which more unifonm heat flux is achieved about the circumference of the process tubes in the furnace chamber.

Still another object of this invention is the provision of a new and better process heater in which the process tubes can be more closely spaced than has heretofor been practical so as to enable more process tubes to be placed in the same sized furnace chamber.

Various other objects and advantages of the invention will become clear in the course of the following description of several embodiments thereof, and the novel features will be particularly pointed out in connection with the appended claims. A better understanding of the invention will be gained by referring to the following description in conjunction with the accompanying drawings, which are illustrative only and are not to be interpreted in a limiting sense, and in which:

FIGURE 1 is a cross sectional elevation of a rectangular process heater built in accordance with the principles of the present invention;

FIGURE 2 is a partial horizontal cross section of the process heater of FIGURE 1 taken along lines 2--2;

FIGURE 3 is a partial horizontal cross section similar to FIGURE 2 and, together with FIGURE 2, illustrating the effect of tube spacing on process tubes of the invention;

FIGURE 4 is a cross sectional elevation of an annular process heater built in accordance with the principles of the present invention; and

FIGURE 5 is a partial horizontal cross section of the apparatus of FIGURE 4.

3,269,3fi3 Patented August 30, 1966 In FIGURE 1, there is shown a process heater built in accordance with the principles of the present invention generally designated by the numeral 10. As shown, process heater 10 may be used as a steam reformer for reacting a steam-hydrocarbon mixture under high temperature (l400 F. to 1800 F.) conditions in the presence of a catalyst to produce free hydrogen and other products of the reaction. The heater I0 is rectangular in shape and has a top wall 12 and a bottom wall 14 manufactured of a refractory material. The reformer 10 also includes side walls 16 and 18 which, with the top and bottom walls 12 and 14 define a heating chamher 29. The side walls 16 and 18 are manufactured of a refractory material and have respective burners 22 and 24 therein. The burners 22 and 24 extend the length and width of the refractory side walls 16 and 18. The purpose of having a plurality of burners is to enable side walls 16 and 13 to approach, as nearly as possible, a uniform radiating plane. The burners 22 and 2.4 are supplied with fuel through conduits 26 and 28 respectively. The heater is supported on suitable structural framing 15.

In chamber 24 there are placed a plurality of vertically extending process tubes 30, 30, etc. Process tubes 30 and 36 are supplied, in the case of a steam reformer, with a steam-hydrocarbon mixture through inlet conduit 32. The inlet conduit 32 feeds a common manifold 34 which supplied the steam-hydrocarbon mixture to all of the tubes 30, 3%, etc. The manifold 32 is connected through a coupler 36 to the tube 30, with each of the tubes having their own individual coupler. The steam-hydrocarbon mixture will react under the high temperature (1400 F. to 1800 F.) conditions in the presence of a catalyst such as nickel oxide to form free hydrogen with carbon monoxide, and other products. After passing through the tube 30, the free hydrogen and other products of the reaction are removed through manifold 38 which is coupled to the tube 30 through a coupler 40. The manifold 38 is connected to an outlet conduit 42.

While the apparatus of FIGURE 1 has been described with reference to steam reforming, it will be obvious to those skilled in the art that it may also be utilized in other operations such as, for example, in simple fluid heating or as a pyrolysis heater. For a particular service, there Will generally be an optimum size and arrangement of process tubes, either horizontal or vertical, and any such arrangement may advantageously employ the novel features of the present invention, as set out fully hereinbelow.

The tubes 30 and 30 are manufactured of a material suitable for the particular service. According to the invention, the tubes 30, 30', etc. each have a pair of vertically extending fins 44, 46 and 44', 46, etc. extending the length thereof (as shown in FIGURE 2). Only the tube 30 with its associated fins 4d and 46 will be discussed in detail, it being understood that the remaining tubes in the process heater are similar thereto. The fins 44 and 46 are located on a center line of the tube 30 parallel to the radiating planes of side walls 16 and 18. As shown, the fins 44 and 46 are one quarter of the tube 30 diameter in width, and one eighth of the tube 30 diameter in thickness; the tubes 30 and 30' are spaced from each other a distance equal to two diameters from center to center.

The positions which are closest to the radiating side walls 16 and 18 are points of maximum heat flux and are indicated at 48 and 50. That is, the side Walls 16 and 18 provide heat flux directly through the burners 22 and 24, respectively, and additionally, reradiate and reflect heat received thereon so that points nearest the side walls 16 and 18 normally receive the maximum heat flux.

In order to estimate the radiant heat flux around the periphery of the tube 30, it was necessary to make certain simplifying assumptions which modify the results somewhat, but not to any significant degree. The two principal assumptions that were made were that (a) the tube walls are gray and (b) that variation in peripheral tube wall temperature is negligible.

In FIGURE 2, the results of calculations based on a four-inch diameter tube have been superimposed on the drawing of tube 30 to indicate the changes in relative radiant heat flux as a function of the position on the periphery of the tube 30. Curve AB indicates the relative radiant heat flux about the tube 30 without the fins 44 and 46. The distances of the points on the curve from the circumference of the circle represented by the outside face of the tube 30 are proportional to the radiant flux to respective points on the tube. Thus, the point 48 directly opposite the radiant wall 16 is shown as receiving radiant heat at an equivalent rate equal to unit. A point 52 on the tube 30 which is thirty degrees away from the point 48 receives radiant heat at a relative rate of only 0.92. A point 54, which is forty five degrees from the maximum point 48, has a relative radiant heat fiux of only 0.82. A point 56, which is sixty degrees from the maximum point 48 has a relative heat flux of only 0.72. Finally, at ninety degrees from the maximum point 48 (at the point of the placement of fins 44) there is only 0.65 relative heat flux.

Curve AC in FIGURE 2 shows the relative heat flux for the tube 30 with the fins 44 and 46 in place. A portion of the curve is shown as a broken line because its position and shape have not been exactly calculated, but it is believed to be substantially correct. Curve AC shows that with the fins 44 and 46 in place the relative heat flux at the fin 44 is 1.0. This is true even at a point only seventy five degrees from the maximum point 48 of the tube 30. In addition to reducing the difference between the maximum flux and the minimum flux from 0.35 for a condition wherein fins 44 and 46 are not present to 0.18 with the fins 44 and 46 in place, the average heat flux to the tube 30 has also been increased significantly by the fins 44 and 46. The average heat flux to the bare tube 30 without the fins 44 and 46 is approximately 0.82 (in arbitrary units) whereas the average heat flux to the fined tube 30 with fins 44 and 46 in place is approximately 0.91. This represents an increase in capacity of at least elevent percent for the finned tube over the plain tube, making no allowance for the expected improvement due to more uniform wall temperature due to the fins.

The fins 44 and 46 are preferably manufactured of a heat conducting material of high emissivity. Thus, the fins 44 and 46 are intended to receive radiant and convection heat from the furnace gases and radiant walls and conduct this heat to the tube 30 wall. In this manner, heat fiow is increased to that part of the tube which receives the least amount of heat in present, unfinned designs.

In FIGURE 3, there is shown a second embodiment of the present invention in which a process heater is similar to heater 10 of FIGURES l and 2, and which has radiating plane side walls 16 and 18'. The only difference between heater 10 and the heater 10 is that the vertically extending tubes 60 and 60 of the steam reformer 10' are spaced only one and three quarters diameters apart, whereas the tubes 30 and 30 of the steam reformer 10 were placed two diameters apart.

The tubes 60 and 60 are exactly alike and therefore only tube 60 will be discussed in detail. Tube 60 has a pair of vertically extending fins 62 and 64 similar to fins '44 and 46 respectively of tube 30. A relative heat flux Curve DE has been superimposed on FIGURE 3, in the :same manner as Curve AB was superimposed on FIG- URE 2, to show the relative heat flux from point 66 to closest to the radiating plane 16' to the point adjacent fin 62, ninety degrees from the maximum point 66, without y fins Present. Curve DE hOWs that the relative heat thereof.

flux varies from 1.0 at maximum point 66, to 0.92 at thirty degree point 68, to 0.82 at forty five degree point 70, to 0.72 at sixty degree point 72, and finally, to 0.60 adjacent ninety degree point 62. It is of interest to note that because of the difference in tube spacing, i.e., tubes 60 and 60' are closer together than tubes 30 and 30, there is a relative heat flux of only 0.60 at the ninety degree position of tube 60 while there is a relative heat flux of 0.65 adjacent the ninety degree position of tubev 30. On the other hand, the relative flux for the portion of the tube between the top of the tube and the sixty degree point seem to be identical in both cases. The reason for this effect is that the portion of the tube between sixty degrees and ninety degrees of tube 60 is hidden from the radiating plane 16' by the adjacent tubes.

Curve DF is the relative heat flux curve for the tube 60 with the fins 62 and 64 in place. This relative heat fiux curve is shown to be very nearly the same as Curve AD of FIGURE 2. Thus, by the use of fins, it is possible to employ a closer tube spacing and still have substantially the same heat flux pattern as with a wider spacing. This provides a substantial saving in the cost of furnace construction because narrower furnaces are made possible.

In the alternative, the same size furnace can have an increased capacity, as more tubes can be placed in the same furnace chamber. It has been found that tubes spaced as little as one and one-half diameters apart have similarly advantageous heat fiux patterns.

It should be clear that the results shown in FIGURES 2 and 3 with respect to the heat flux curves apply to the fin sizes which were used and that the shape of the heat flux curve will vary with the choice of sizes of fins. The heat flux curve will also change with the relative heat conductivity of the fins and the rate of heat absorption of the fins. The preferred range of fin sizes is from one eighth the tube diameter in thickness to three eighths the tube diameter in thickness and from one eighth the tube diameter in width to one half the tube diameter in width, but optimum fin sizes must be determined for any given installation.

In FIGURE 4, there is shown a pyrolysis heater 74, built in accordance with the principles of the present invention. The pyrolysis heater 74 has a cylindrical outer wall 76 with a refractory lining 78 and a cylindrical inner wall 80 with a refractory liner 82. The outer wall 76 and inner wall 80 define an annular chamber 84 therebetween. The outer wall 76 is supported on suitable structural steel members 85. Centrally disposed within the inner wall 80 there is a convection section 86 and a stack 88. Suitable ducts 90 provide a passage for combustion fumes from the annular chamber 84 into the convection section 86 and stack 88. The operation of the annular furnace 74 is more fully described in my copending application Ser. No. 384,706, filed July 23, 1964, and entitled Apparatus.

The inner wall 80 has a plurality of burners 92 along the length and width thereof. Also, the outer wall 76 has outer wall burners 94 along the length and width The burners 92 and 94 are intended to heat vertically extending process tubes 96 centrally disposed in a circular path within the annular chamber 84 although other tube configurations may be employed. As shown, the tubes 96 are spaced equidistant the inner wall 80 and the outer wall 76. The process tubes 96 are intended for a pyrolysis heater such as one utilized in the production of ethylene. The fuel for the process is supplied through an inlet conduit 98 and removed through an outlet conduit 100. The pyrolysis heater has reverse bends 102 for permitting a plurality of passes of the process fluid through the process tubes 96.

The process tubes 96 have vertically extending fins 104 and 106 on opposite sides thereof extending the length of the tubes 96. The fins 104 and 106 are similar to the fins 44 and 46 of tube 30 and the fins 62 and 64 of tube 60. These fins are attached at the points on tubes 96 where the arc of the circular path of the tubes 5 96 intersects the walls of the tube 96. The fins 104 and 106 perform the same functions described with respect to the rectangular furnaces in that they achieve a more uniform heat flux about the circumference of the process tubes and enable the process tubes to be more closely spaced together.

It Will be understood that various changes in the steps, materials and arrangements of parts, which have herein been set forth to describe and illustrate the invention, may be made by those skilled in the art within the scope of the invention as defined in the appended claims.

What is claimed is:

1. A process heater comprising,

a housing having a heating chamber therein,

said housing including two parallel spaced radiating surfaces for heating said chamber,

a plurality of process tubes within said chamber in spaced relation to said radiating surfaces and to each other;

two diametrically opposed elongated fin means integral with said process tubes placed along the line furthest from both radiating surfaces, said line being the line of approximate minimum heat flux to said tubes from said radiating surfaces;

each said fin means having a thickness ranging between one-eighth and three-eighths of the tube diameter and a width ranging from one-eighth to one-half of the tube diameter; and

means for passing process fluid through said tubes.

2. In a process tube for use in a process heater, the improvements comprising integral, diametrically opposed fin means along the length of said tube, each of said fin means having a thickness ranging from about one-eighth to about three-eighths of a tube diameter and a width ranging from about one-eighth to about one-half of a tube diameter.

3. The process heater as claimed in claim 1, wherein the spacing between said tubes is approximately one and one-half to two tube diameters from center line to center line of each tube.

References Cited by the Examiner UNITED STATES PATENTS 2,081,971 6/1937 Alther 122356 X 2,108,397 2/1938 Watts 122-356 X 2,638,879 5/1953 Hess 122356 2,751,893 6/1956 Permann 122335 X CHARLES I. MYHRE, Primary Examiner. 

1. A PROCESS HEATER COMPRISING, A HOUSING HAVING A HEATING CHAMBER THEREIN, SAID HOUSING INCLUDING TWO PARALLEL SPACED RADIATING SURFACES FOR HEATING SAID CHAMBER, A PLURALITY OF PROCESS TUBES WITHIN SAID CHAMBER IN SPACED RELATION TO SAID RADIATING SURFACES AND TO EACH OTHER; TWO DIAMETRICALLY OPPOSED ELONGATED FIN MEANS INTEGRAL WITH SAID PROCESS TUBES PLACED ALONG THE LINE FURTHEST FROM BOTH RADIATING SURFACES, SAID LINE BEING THE LINE OF APPROXIMATE MINIMUM HEAT FLUX TO SAID TUBES FROM SAID RADITAING SURFACES; EACH SAID FIN MEANS HAVING A THICKNESS RANGING BETWEEN ONE-EIGHTH AND THREE-EIGHTHS OF THE TUBE DIAMETER AND A WIDTH RANGING FROM ONE-EIGHTH TO ONE-HALF OF THE TUBE DIAMETER; AND MEANS FOR PASSING PROCESS FLUID THROUGH SAID TUBES. 